Mechanical Properties of Douglas Fir Wood at Elevated Temperatures under Nitrogen Conditions

The mechanical properties of wood tend to decrease with increasing temperature under normal atmospheric conditions. A pyrolysis zone develops inside wood when it catches fire, and the surface is charred; it is important to understand the mechanical properties of the wood under the charred surface. In this study, the mechanical properties of Douglas fir wood, such as its compressive, tensile, and bending strengths, were measured under nitrogen atmosphere at nine temperatures between 20°C and 280°C for exposure times ranging from 60 to 120 min. The results indicated that the wood's mechanical properties under the nitrogen atmosphere (i.e., oxygen-depleted conditions) decreased with increasing temperature, and the exposure time had little effect on the investigated properties. The mechanical properties of the wood under the charred surface exhibited a nonlinear decrease with increasing temperature due to the hydrolysis reactions. The mechanical properties were accurately described by temperature-dependent equations combining a linear model with three polynomial functions. Scanning electron microscopy revealed that high temperatures in the oxygen-free environment induced severe microstructural damage to the wood.